The present invention relates to Automatic Power Controller (APC) with which the intensity of light emitted from a semiconductor laser used in a laser printer or the like is controlled to a set value.
Since the intensity of light emitted from a semiconductor laser has a tendency to be strongly affected by the ambient temperature, the intensity of the light emitted from the semiconductor laser is detected at the start of the printing of every page, for example, and an adjustment process is effected to adjust the semiconductor laser drive current so as to keep the intensity at a set value. Adjustment of the intensity is stopped when the detected value of the intensity of the emitted light reaches the vicinity of the set value.
However, since the photosensitive drum in a laser printer must be charged with static electricity by a corona discharge, an automatic power controller used to control the intensity of the light emitted from the semiconductor laser is likely to be affected by noise. Hence, it often occurs that the control process is stopped due to an error signal despite the fact that the intensity has not reached the region of the set value. In the case of stopping the control process due to such an error signal, the printing density may be too low or too high, or an overcurrent may flow through the semiconductor laser, causing thermal runaway and damaging the laser diode.